1. Field of the Invention
This invention relates to apparatus and methods for seismic prospecting, and, more particularly, to an electronic roll-a-long switch and method of using same.
2. Description of the Prior Art
In seismic exploration, sound waves are commonly used to probe the earth's crust as a means of determining the types and location of subsurface formations. The earth's crust can be considered a transmission medium or filter whose characteristics are to be determined by passing sound waves through that medium. In the reflection seismic method, sound disturbances are made at a transmission point at or near the earth's surface, and sound waves which are reflected from subsurface reflecting boundaries are detected by seismic detectors, e.g., geophones, which generate electrical signals at their outputs. Information relating to subsurface formations is contained in these electrical signals, and the signals are recorded in a form which permits analysis. Skilled interpreters can discern from the analysis the shape and depth of subsurface reflection boundaries and the likelihood of finding an accumulation of minerals, such as oil and gas.
In a typical seismic field recording system, the arrays of seismic detectors are located at regularly spaced intervals along that portion of the earth's surface under consideration. A typical seismic exploration system also includes a recording truck and and a multi-pair cable, which is used to connect the outputs of the detector arrays to the recording truck. Typically, a pair of wires is "taken out" of the recording cable for appropriate connection to the output of each array. The location at which a given pair is taken out of the cable has been referred to by such designations as "receiver point", "receiver station", or "station". In this application such a location is designated a "receiver point". Typically, the receiver points are given sequential numbers, e.g., from 1 to N, where N is the total number of receiver points in the system. The data obtained at each receiver point from the output of an array of detectors forms a "channel" of information, which is provided to the recording apparatus.
In a typical seismic system, the total number of receiver points exceeds the channel capacity of the recording apparatus in the recording truck. Accordingly, data are not recorded from all receiver points in response to each seismic disturbance. Rather, responsive to each disturbance, data are recorded from a number of receiver points equal to the channel capacity of the recording equipment in the recording truck. The length of time that data are recorded responsive to a seismic disturbance is known as the record interval, and a typical record interval may be on the order of 6 to 10 seconds. During a record interval time, the outputs of the receiver points providing data to the recording system are sampled periodically and the samples are recorded. The time between successive samples on the same channel is called the channel sample time. It is common for the channel sample time to be 1 millisecond, 2 milliseconds, or 4 milliseconds. Thus, during a record interval of 6 to 10 seconds, data are sampled and recorded for each channel a substantial number of times.
Included in the recording truck is apparatus which selects the group of receiver points from which data are to be recorded during a given record interval. Such apparatus is commonly referred to as a "roll-a-long switch". Heretofore, roll-a-long switches have generally been mechanical in construction. That is, mechanical switches having contacts have been used to implement roll-a-long switches in prior art devices. These mechanical switches are subject to damage through wear and due to extreme environmental conditions, corrosion, and dust. All of the above factors adversely affect the ability to obtain accurate seismic data.
The shortcomings of such prior art roll-a-long switches have been overcome with the present invention.